Fuel systems for engines may include pumping devices configured to pressurize the fuel prior to injection into the combustion chambers of the engine. For example, in common rail fuel systems, a tappet assembly may be configured to drive a plunger and/or piston, which may be configured to pressurize the fuel. The tappet assembly may include a tappet having a pin attached to the tappet and a roller mounted about the pin, and configured to rotate around it. The roller may be configured to contact a cam lobe, which drives the tappet up and down.
For heavy duty applications, the loads on such tappet assemblies may be significant, which can cause failure of one or more components of the assembly if the assembly is not constructed robustly enough. In some cases, the effect that certain loads have on the assembly can be amplified by stress concentrations. For example, in some assemblies, stresses can become concentrated at the ends of the roller. Areas of stress concentrations can act as the weakest link in an otherwise robust assembly, leading to seizing and/or cracking of rollers.
Some assemblies have been developed that attempt to reduce stresses. For example, U.S. Pat. No. 2,735,313, issued to Dickson (“the '313 patent”), discloses a roller having a crowned inner surface and a crowned outer surface. The crowned inner surface allows for a more even load distribution on the inner surface of the roller and/or the outer surface of the pin. The crown is designed to mate better with the pin under loading. Under loading, the pin bends, conforming the pin with the crown of the inner surface of the roller.
The outer surface of the roller is crowned in such a way as to roll on a similarly but oppositely crowned camshaft follower lifting surface, allowing for unrestrained rolling engagement during cocking of the roller about its shaft 20. In other words, because of the crowned inner surface of the roller, under light loads when the pin does not bend, the roller may tilt back and forth relative to the pin. In order to maintain suitable rolling contact with the tilted roller, the outer surface of the roller and the camshaft follower lifting surface have been crowned to facilitate rolling engagement.
In addition, poor frictional properties of mating components may also lead to failure, particularly during engine start-up when lubrication oil may not have been circulated yet. Surfaces of the tappet assembly must not only possess significant strength, but also must have low frictional properties. Some assemblies have provided coatings, such as tungsten carbide carbon (WCC), on various surfaces of the assembly to create a low friction, durable surface on top of a high strength material, such as steel. However, none of these assemblies have utilized a coating such as WCC on the outer surface of the roller.
While the device disclosed in the '313 patent may disclose a configuration designed to reduce stresses between the pin and roller, the interface between the roller and camshaft follower lifting surface is not configured such that a footprint of contact pressure from the camshaft follower lifting surface, at maximum operational loading conditions of the machine, is spread substantially the full width of the outer surface of the roller. Therefore, in the device of the '313 patent, the contact patch between the roller and the camshaft follower lifting surface is relatively narrow, even at high loads. Concentration of high loads in such a narrow contact patch results in high stresses that may render the roller susceptible to failure.
The present disclosure is directed at improvements in existing tappet assemblies.